The invention relates to a device for monitoring at least one parameter for a plurality of vehicle wheels.
The monitoring of parameters of a vehicle wheel, for example, of the tire pressure or the temperature of the tire, plays a decisive role for the safety of a vehicle or of the driver of a vehicle. To be able to dispense with manual checking of such parameters, devices have been developed with which it is possible to register important parameters of vehicle wheels automatically and display them, for example, by appropriate display devices on the dashboard. As it is generally desired not only to display the parameters but also to indicate a uniquely defined assignment between the displayed parameters and the position of the respective wheel, it is necessary to configure such a device to retain the assignment, or to re-initialize it, after a wheel is changed.
For such a purpose, existing in the prior art is an assignment of each detector device respectively disposed on a wheel to a receiver unit that is provided in the vicinity of the respective wheel position. As such, it is possible to detect the signal of the respectively adjacent detector device by reference to the intensity of the signals received through a receiver unit, by selecting the signal with the greatest intensity. It is assumed here that the detector devices of the wheels of a vehicle transmit with substantially the same transmitting power so that the signal of the respectively directly adjacent wheel or of the adjacent detector device has the greatest signal intensity at the reception location.
However, a disadvantage with such a device is that a receiver device is necessary for each wheel, the received signal of such a receiver device then having to be conducted in line-bound fashion to a central evaluation and control unit or directly to a display unit.
European Patent Application EP 0 806 306 A, corresponding to U.S. Pat. No. 5,808,190 to Ernst, discloses an air-pressure monitoring system in which the assignment of the wheel positions to the air-pressure monitoring devices that are disposed on the wheels is made possible by the fact that a further parameter is registered for each wheel with the air-pressure monitoring devices and with, in each case, a further measuring device assigned to the wheels. The parameter may be, for example, the rotational speed of the wheel. The air-pressure monitoring devices, thus, transmit not only measured values for the air pressure but also measured values for the further parameter to a central unit. As measured values are also transmitted from the further measuring devices to the central unit for the same further parameter, the central unit can make an assignment of an air-pressure monitoring device to a wheel position by comparing the measured values for the further parameter, if the measured value supplied by the respective air-pressure monitoring device for the further parameter coincides sufficiently precisely with the respective measured value of the parameter of a further measuring device. This is because the further measuring devices are disposed fixedly on the vehicle (not on the wheel), and, thus, remain continuously assigned to a specific wheel position.
A disadvantage with such a device is the additional expenditure on the two sensors for registering the further parameter. Even if the further parameter can be registered with the sensor that is provided in any case on the wheel for registering the air pressure, a further sensor that is provided fixedly on the vehicle is necessary for each wheel in order to register the further parameter.
Finally, European Patent Application EP 0 931 679 A discloses a method for assigning the wheel position of a motor vehicle in which the transmission signals of transmitter units assigned to the wheels are transmitted to a central receiver and evaluation unit, and in which the different characteristic influences of the individual transmission paths on the respective reception signal as the wheels rotate are widened in order to assign the signals to the wheel positions. For such a purpose, the envelope curve of the reception signal that is amplitude-modulated by the time variants including changes in the transmission properties is compared with stored signatures. However, the relatively complex registration and evaluation of the envelope curve is a disadvantage here.
It is accordingly an object of the invention to provide a device for monitoring at least one parameter for a plurality of vehicle wheels that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that makes it easily possible to assign the detector units disposed on the wheels to the wheel positions with the necessary reliability and, at the same time, to achieve the lowest possible expenditure on hardware.
With the foregoing and other objects in view, there is provided, in accordance with the invention, a device for monitoring vehicle wheels, including detector units disposed at the wheels, each detector unit having a sensor for registering at least one parameter of a wheel and a transmitter unit transmitting information regarding the at least one parameter, each of the detector units transmitting, with the transmitter unit, the information in a modulated signal over a wireless transmission path, a central evaluation and control unit having a receiver unit receiving the information from the transmitter unit and demodulating the signal, the control unit evaluating the information from the detector units, and generating signals dependent thereon, the control unit having a normal operating mode and an assignment mode and, in the normal operating mode each detector unit transmitting, as the information, an information item relating to the registered parameter and a wheel identifier uniquely defined for each of the wheels and the control unit evaluating the wheel identifier and the information relating to the registered parameter from the signal demodulated by the receiver unit and assigning a wheel position to each information item relating to the registered parameter from the identifier and from a stored assignment information item, in the assignment mode each the transmitter unit transmitting, as the information, at least the wheel identifier, the receiver unit generating a signature signal representing a measure for an average signal power of the received modulated signal, a duration of the signal transmitted by the transmitter unit of the detector units being shorter in comparison with a time required for a complete revolution of a wheel at a maximum speed of the wheel, the control unit registering the signature signal derived from each received signal and assigning an angle information item to the respective value of the signature signal, the control unit acquiring the angle information item from one of a time interval between one of the signature signals and the signature signals and a fixed time, and the group consisting of a velocity of the vehicle, a parameter proportional to the velocity of the vehicle, and an angle information item relating to an angular position of the respective wheel fed to the control unit, and the control unit assigning to the wheel position the wheel identifier transmitted by each of the detector units by comparing one of registered values and an information item determined therefrom with a stored signature information characteristic of each the wheel position, and storing the angle information item as an assignment information item.
The invention is based on the recognition that when data is transmitted by a preferably phase-modulated or frequency-modulated signal, the signal power occurring at the reception location of a central evaluation and control unit is influenced by the properties of the transmission path between the detector unit disposed on a wheel and the evaluation and control unit. The properties of the transmission path are determined here, on one hand, by the angular position of the wheel, and, thus, the position of the detector unit, and, on the other hand, by the parts or regions of the vehicle that influence the signal transmitted from the respective angular position. It has been found in practice that as the wheels rotate, the transmission paths at the various wheel positions are respectively different and at the same time characteristic of the wheel positions.
According to the invention, the mean signal power of the received signal is, therefore, evaluated by the evaluation and control unit to permit a detector unit to be assigned to a wheel position. For such a purpose, in an assignment mode, the evaluation and control unit compares values of the signature signal that are registered at various angular positions of the respective wheel, or an information item derived therefrom, with a stored signature information item that is characteristic for each wheel position.
The device according to the invention, therefore, permits the necessary assignment of the detector devices to the wheel positions, exclusively by a specific evaluation of a phase-modulated or frequency-modulated signal that is present in any case.
It is to be noted here that, during the determination of the mean signal power, the carrier frequency of the modulated signal is filtered such that the mean value that is determined can still follow the (relatively slow) change due to the revolution of the wheel.
Signature information is understood here to be any information, even any specific way of representing the registered values of the signature signal.
According to the invention, the duration of the signal transmitted by a transmitter unit is short in comparison with the time required for a complete revolution of a wheel at a maximum permitted or maximum possible speed. The evaluation and control unit assigns an angular information item to the discrete signature signal values that are generated thereby. The discrete values of the signature signal for different angular positions of the respective wheel are, thus, generated by the respectively chronologically shorter transmitted signals.
The assignment of the angular information can be carried out by supplying the evaluation and control unit with the signal of an angular sensor that registers the rotation of the respective wheel.
Instead of one angular sensor per wheel, it is also possible to provide merely a single angular sensor that registers the rotation of a wheel, its signal being used as an angle signal for all the wheels. This simplification will be permissible in most cases as the angular positions of the individual wheels (apart from deviations due to slip or slightly different rotational speeds when cornering) are largely identical.
In these embodiments, the evaluation and control unit can, if appropriate, also receive information about the absolute angular position of the respective wheel. However, this is not absolutely necessary for most embodiments of the invention that are explained below.
All that is necessary is for the evaluation and control unit to know the angular spacing between two received signals. One embodiment of the angular sensors as an incremental signal transmitter is sufficient in this case.
The evaluation and control unit can also acquire the angle information from the time interval between the signature signals or the time interval between the signature signals and a fixed time and from the velocity of the vehicle or a parameter that is proportional thereto. For example, the detector devices could transmit signals at equidistant time intervals in the assignment mode, and a signal that is a measure of the velocity of the vehicle or the rotational speed of the wheels could be fed to the evaluation and control unit. It is then possible to determine the angular spacing from the velocity of the vehicle or the rotational speed of the wheels, using the time interval between two signals.
When the angle information is assigned, it is, in principle, not necessary for one angle value to be assigned explicitly to one value of the signature signal. Instead, the process can also be carried out in that, for example, an angle value or a range of angle values is represented by a specific storage location or storage address.
In accordance with another feature of the invention, the stored signature information item is a characteristic profile that is formed from values of the signature signal acquired in a predetermined fashion in a reference mode at a plurality of angular positions, a first partial information item being made dependent on a second partial information item. Instead of such a profile, the stored signature information can also merely include one or more characteristic variables of such a profile.
The evaluation and control unit can, of course, also replace the discrete profile with a single function or in certain sections by a plurality of analytical functions.
For example, the stored signature information can be a characteristic (discrete or continuous) profile of the signature signal as a function of the angular position or of a variable connected thereto, it being preferably possible for the angular range to be one revolution of a wheel.
The stored signature information can also be present as histogram-like information, the value range for the signature signal being divided into sections of preferably equal width, and, in each case, the registered signature signals that occur in such a section being stored. The evaluation and control unit must then have the information indicating how the stored values have been registered with respect to their angular spacing and must perform the registration of the signature signal values in the same way, so as to permit comparability of the registered information and of the stored information.
The evaluation and control unit can also determine one or more characteristic variables of a profile that is formed from a plurality of signature signal values that are registered at specific angular positions of the respective wheel, and compare them corresponding stored characteristics variables or with characteristic variables determined from corresponding stored profiles. Such characteristic variables may be the mean value, the standard deviation or the maximum difference between the registered signature signal values or the stored profile of the signature information.
In accordance with a further feature of the invention, the stored signature information is a characteristic profile of the signature signal as a function of one of an angular position and a variable connected thereto.
In accordance with an added feature of the invention, the control unit determines at least one characteristic variable of a profile formed from a plurality of signature signal values registered at specific angular positions of a respective one of the wheels and compares the at least one characteristic variable with one of corresponding stored characteristic variables and characteristic variables determined from corresponding stored profiles.
In accordance with an additional feature of the invention, the evaluation and control unit can carry out a cross-correlation of the determined profile of the signature signal with the stored signature profiles and exclude those wheel positions that are assigned to those signature profiles at which the correlation coefficient is smaller than a predefined limit or assign those wheel positions that are assigned to those signature profiles at which the correlation coefficient is greater than a predefined limit.
In accordance with yet another feature of the invention, the evaluation and control unit compares a value pair composed of a signature signal and the associated angular position with all the stored signature profiles, and excludes from the assignment those wheel positions that are assigned to those signature profiles at which the comparison of the signature signal with the value of the signature profile at the respective angular position results in a deviation that is greater than a predefined limit. Of course, the evaluation and control unit can also make, as a function of the comparison, an assignment of those wheel positions at which the comparison of the signature signal with the value of the signature profile at the respective angular position results in a deviation that is smaller than a predefined limit.
The evaluation and control-unit preferably compares as many value pairs of one and the same detector device, or of different detector devices, with stored signature profiles as are necessary for a uniquely defined assignment to a wheel position to be obtained for all the detector devices.
However, for such a purpose, it is necessary for the evaluation and control unit to register or know the absolute angular positions of the wheels. In addition, the wheels must always be mounted in the same angular position on the hub, or the evaluation and control unit knows the possible angular differences between the possible mounting positions (as a rule dependent on the number of attachment screws distributed equidistantly over the circumference) and attempts to test all the possible positions by adding the possible difference angle.
In the preferred embodiment, the detector devices switch over into the assignment mode if no revolution of a wheel is detected for a time period that is longer than a predefined value. The predefined value for the time period can be selected, for example, to be approximately equal to the time that is necessary for a wheel change. The detection of whether or not a revolution of a wheel takes place can be carried out, for example, by an acceleration sensor that is included in the detector unit. In the assignment mode, the detector units transmit signals repeatedly and at different angular positions.
In accordance with yet a further feature of the invention, the control unit has a reference mode in which the control unit acquires the signature information by evaluating and storing the signals transmitted by the detector unit and the control unit is informed of an assignment of a signature profile to a respective the wheel position through one of externally supplied signals and externally supplied data inputs.
In accordance with a concomitant feature of the invention, the evaluation and control unit for determining the stored signature profiles or stored characteristic variables can be set to a reference mode. In such a mode, the evaluation and control unit picks up the signature profiles by evaluating and storing the signals transmitted by the detector units. Which signature signal is assigned to a particular wheel position is communicated to the control unit here manually or by external signals. The communication can be done, for example, by communicating the identifier of each of the detector units and the wheel position at which the respective detector unit is located to the control unit.
Other features that are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a device for monitoring at least one parameter for a plurality of vehicle wheels, it is, nevertheless, not intended to be limited to the details shown because various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.